1. Field of the Invention
The invention is a new technique for measuring the velocity of ultrasonic surface waves (USW) propagated on the surface of a test specimen which has been provided with two parallel scratches spaced apart a precisely known distance, for the purpose of measuring the elastic mechanical parameters of the specimen near its surface by applying an external static stress (which may be zero or non-zero) on that surface, in order to determine the residual stress and the mechanical strength properties of the specimen.
2. Description of the Prior Art
Inasmuch as the accuracy of the USW absolute velocity measurements depends upon a precisely known distance traversed in a precisely measured time interval between generation and detection of the USW signal, it follows that an imprecise determination of said distance, and coupling variations between the transducer and the specimen surface will greatly limit the accuracy of the USW velocity measurements.
Prior Art methods for measuring USW velocity are extremely accurate for making relative measurements when the propagation distance and the applied external stress state are constant but not necessarily known. On the other hand, an absolute velocity measurement requires precise knowledge of the propagation distance, and constant coupling conditions. A major problem in prior art methods is the difficulty which those two limitations impose upon the accurate measurement of absolute USW velocity measurements. Consequently, it is extremely difficult to measure accurately either the second or third order elastic constants of the specimen.
Moreover, the coupling variations become completely unmanageable when the USW velocity measurements are made in the presence of an externally applied stress that causes a relative strain or deformation of the coupling agent at the interface between the specimen and the transducer. Another problem is the difficulty of precisely determining the actual propagation distance between the two transducers, because a transducer is not a point source and the exact origin of the USW in the transducer is difficult to determine accurately.
Another prior art method is described in: Elements of X-Ray Diffraction, by B. D. Cullity, copyright 1956, chapter 17, "Stress Measurement", published by Addison-Wesley Publishing Co., Inc.; SAE undated Pub. No. TR-182, "Measurement of Stress by X-Ray", by Christensen, ed, Koistinen and Marburger, GMC, Semchysen, Climax Moly, and W. P. Evans, Caterpillar; and GM Res. Labs. Pub. GMR-825, "GMR FasTress: A Fast Automatic Stress Analyser," 1968.